Engraving and scanning apparatus

ABSTRACT

An engraving head incorporates a shaft which is supported for high frequency oscillation by axially spaced metal spring elements, and the shaft is acted upon by improved dampening means to minimize undesirable transverse and transient vibrations. A diamond cutting stylus is carried by a holder mounted within an arm projecting from the shaft by means which facilitates convenient removal and resharpening of the stylus. A diamond guide shoe has a flat face which engages the surface to be engraved adjacent the stylus and is supported by means which may be conveniently adjusted without rotation to obtain precision alignment of the face with the surface. A diamond deburring or shaving element has a shearing edge adjacent the stylus and is also supported for convenient angular adjustment to obtain precise alignment. The entire engraving head is pivotally supported by leaf springs and is raised and lowered by power operated means. The apparatus also incorporates a scanning head including an improved optical system for precisely and effectively illuminating and detecting the copy to be engraved. The apparatus further includes a control system which automatically corrects and compensates for unbalance in copy density at repeated lengths on the copy, and the engraver shaft drive is controlled to produce a flat frequency response which passes through the resonant frequency of the oscillating components.

This application is a division of Ser. No. 56,623 filed July 11, 1979,now U.S. Pat. No. 4,357,653.

BACKGROUND OF THE INVENTION

This invention relates to the engraving of cylinders commonly used inthe gravure printing process, and specifically to engraving apparatus ofthe general type disclosed, for example, in U.S. Pat. Nos. 2,881,246,2,874,479, 3,964,382 and 4,013,829. The basic principle ofelectro-mechanical engraving of a gravure cylinder involves rotating acopper plated cylinder while actuating an electrically driven tool whichcuts or engraves cells or lines into the copper surface. The engravedcylinder is normally used in a web type gravure printing press forprinting paper, plastic, or metallic film material.

In the gravure printing process, the engraved cylinder is flooded withink, and a doctor blade wipes off excess ink from the surface so thatonly the engraved cells contain ink which is transferred to the materialbeing printed. To obtain a high quality print, it is necessary that thecells be very accurately placed or located on the cylinder surface,usually within one to two microns of the desired theoretical location.The depth of the engraved cells must also be accurately controlled sincethe the depth determines the amount of ink transferred which, in turn,determines the shade of gray in a black-white print. In a color print,the amount of ink transferred to the paper or material is even morecritical since three colors are mixed to produce various shades of allpossible colors. A slight variation in the desired amount of ink effectsnot only the darkness of the color but, more importantly, the productionof the desired color tone.

In addition to printing newspapers and magazines, the engraved cylindersmay also be used for direct or indirect printing of cloth, applyingglue, printing of packaging materials for products, and printing of woodgrain patterns for making of wall paneling, floor coverings and othersurface coverings.

The cutting tool used to engrave the cells is normally a pointed diamondstylus. Other tools made of sapphire, carbide, cobalt steel, etc. mayalso be used, but generally give shorter life, and due to wear, do nothold as consistent a point as diamond. The tool must make many cells ina cylinder, and therefore, must be operated at a very high speed. Forexample, in a typical 140 line screen, about 20,000 cells per squareinch are required. More than 100 million cells are frequently requiredfor a single large diameter gravure printing cylinder. Even with aforming rate of 3,000 to 4,000 cells per second, several hours of timemay be required to engrave a single cylinder. Such a high cell formingrate introduces serious problems of high acceleration forces withresulting torsional and transverse or lateral vibrations. It is alsonecessary to make rapid transfer from black to white (full cells to nocells) or white to black. This also introduces transients causingserious torsional and transverse vibrations.

In the engraving of a gravure cylinder, the image pattern or copy to beengraved is usually mounted on a copy cylinder, and the copy isoptically scanned while the engraving is being performed. However, thecopy may be scanned and the corresponding information stored in computermemory, processed, and later used to engrave a cylinder. The engravingmachine may be an electro-mechanical engraver which uses a diamondstylus to engrave the cylinder, or the machine may incorporateelectronic means such as electron beam or laser for forming the cellswithin the cylinder. In either machine, a series of cavities and/orlines are engraved into the cylinder surface. The cavities are adaptedto carry ink which produces the image on the material being printed. Theimage may involve either very small images such as printing typerequiring very small and well defined lines or pictures requiring veryclose control of different cylinders for different colored inks neededfor close color matching or large images for printing items such aswallpaper.

In electro-mechanical engraving apparatus, there are a number ofspecific problems. For example, there is a strong tendency for themechanical stylus holder and its spring support system to "ring" orvibrate rotationally thereby producing "ghost" images which aredisplaced from the main image. This greatly reduces the quality of theprinting, especially of small type, and dampening means must be used ina form which does not introduce hystersis. The cost of sharpening thediamond engraving stylus is also very high due to the necessity forremoving the stylus from its holder in order to grind the cuttingsurfaces.

There are also problems presented by the bearing or bearings commonlyused to support the actuating shaft which carries the cutting tool. Abearing introduces friction, unpredictable hysteresis, anduncontrollable lateral or transverse movement of the cutting tool due tobearing looseness. Even with high strength connections between the poweractuated driver and the cutting tool, harmonic vibrations are generallypresent in the coupling shaft and must be avoided or minimized toproduce accurate placement of the cells as well as cells with preciselycontrolled depths. The stylus holder should also be constructed so thatthe cutting tool or stylus may be conveniently replaced and preciselyaligned without extensive adjustment procedures.

Presently used electro-mechanical engraver systems commonly use a roundor ball surface on the diamond guide shoe and thereby avoid the need forprecision alignment of the shoe. However, when a round shoe surfacepasses over a deep engraving, the shoe presses into the copper surfaceand reduces the quality of the first engraving. While it is necessaryfor the diamond shoe to be adjustable, the adjustment in presently usedmachines is provided by mounting the diamond shoe on the end of a finepitch screw. As a ball point diamond wears, it takes on a slightconcaved shape. Thus when the diamond shoe support screw is rotated foradjustment, two lines are drawn on the cylindrical copper surface of thegravure cylinder. These double lines are very undesirable as they maycarry ink and appear in the copy printed by the engraved cylinder.

Another problem is presented by the engraving operation producing copperchips which are always present in the vicinity of the cutting stylus andthe sliding guide shoe. When these chips pass under the shoe, the shoeand engraving head are forced upwardly causing a lesser depth row ofcells and, in turn, a line in the final printed material which may befrom a fraction of an inch in length to several inches in length. Whileforced air is often used to clear chips, further means are desirable toavoid chips passing under the guide shoe.

In the scanning of the image of the copy on the scanning side of theengraving system, there is a problem of obtaining the same reading froma white area surrounded by a dark area as from an equally white areawhere the whole area is white. This is commonly referred to as lighttunneling in the copy material. Light enters the paper or photographiccopy material, is transmitted laterally in the copy material, and someof the light extends a small fraction of an inch from where it enters.It is not uncommon to have a reading from a small white spot surroundedby black which is 20 to 40% less than the reading from an equally whitespot in a white area. Since this reading controls the engraving depth,it is a serious problem in many copy materials. To avoid this problem,it is desirable to have the illumination spot of light as near aspossible to the size of the spot being read.

Since the copy material on the copy support cylinder is not always ofuniform thickness and/or may contain wrinkles, it is necessary that theillumination and pickup system have a good depth of focus. The pickup orreading system normally has a good depth of focus, for example, 1/6inch, by having a high f lens number such as f8 to f22. However, sincethe illumination must come in at a large angle, i.e., 20 to 60 degrees,to avoid direct surface reflections, the illumination effectively comesfrom a lens with low f numbers such as f1 to f2. It is difficult toobtain a good depth of focus with such an illumination spot.

Presently used electro-mechanical engravers provide for manually raisingand lowering the engraving head relative to the cylinder being engraved.However, if the head is not lowered carefully, the diamond stylus may bedamaged, and such damage will not become apparent until after engravingthe cylinder, resulting in a defective or damaged cylinder. Also in the"step and repeat" process of engraving a cylinder after each engraving"step", there is a period of no engraving while the scanner is beingreturned to the start position. If the engraving head is not raisedduring the time of no engraving, the guide shoe of the engraver runs ina single track around the cylinder and may damage the cylinder surface.

SUMMARY OF THE INVENTION

The present invention is directed to improved apparatus and method forelectro-mechanically engraving printing plates or cylinders and whichprovides for effectively solving the above defined problems and formaking electro-mechanical engraving of gravure printing cylinders morecompetitive with chemical engraving of the cylinders.

More specifically, the apparatus and method of the invention provide forminimizing the transverse or lateral vibrations within the engravingstylus holder so that the engraved cylinder produces higher qualityprinting. The invention also provides for conveniently removing theholder for the diamond engraving stylus and for regrinding or sharpeningthe stylus while it remains on the holder, thereby simplifying theregrinding operation and minimizing the downtime of the engravingmachine. The apparatus of the invention also eliminates the use of oneor more bearings for supporting the stylus holder shaft and therebyeliminates the problems associated with such bearings.

As will also become apparent, the apparatus of the invention provides aguide shoe which has a flat face for engaging the cylinder surface and alead edge which prevents chips from passing between the guide shoe andthe cylinder surface. The apparatus also provides for precisely andconveniently adjusting the cutting stylus, the guide shoe and a cleaningshoe relative to the cylinder surface, and for automatically raising andlowering the engraving head at a controlled rate which avoids thepossibility of damaging the cylinder surface. In addition, the inventionprovides a copy scanner with a combined lens and illumination systemwhich assures more accurate scanning or reading of the copy material.

While the invention is illustrated and described in reference to theengraving of a gravure printing cylinder, the apparatus of the inventionmay also be used for engraving flat printing plates, and for whicheither the engraving head or the flat plate is movable. Furthermore,other features and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electro-mechanical engraving headconstructed in accordance with the invention and illustrating itsengraving position relative to a cylinder positioned to be engraved;

FIG. 2 is an elevational view of the front face of the engraving headand showing a portion in section to illustrate internal assembly;

FIG. 3 is a fragmentary section taken generally on the line 3--3 andshowing a portion of the electro-magnetic drive system within theprinting head;

FIG. 4 is an enlarged diagrammatic view illustrating the relativepositions of the diamond guide shoe, diamond cutting stylus and thediamond cleaning or deburring element;

FIG. 5 is a fragmentary perspective view in part-section of the cuttingstylus support assembly and also illustrating a modified form forsupporting the stylus actuating shaft;

FIG. 6 is an enlarged perspective view of the diamond cutting stylusshown in FIG. 5 and projecting from its holder;

FIG. 7 is a perspective illustration of a system for dampeningvibrations in the stylus support shaft;

FIGS. 8-10 are axial sections diagrammatically illustrating modifieddampening systems constructed in accordance with the invention;

FIG. 11 is a somewhat diagrammatic illustration of the copy illuminatingand reading system used within the copy scanning head;

FIG. 12 is a block diagram of the controls for operating the engravinghead;

FIG. 13 illustrates a typical copy having a repeat wood grain patternand showing a typical joint within the pattern; and

FIG. 14 is a block diagram illustrating the control circuit for theengraving head in order to produce a substantially invisible joint onthe engraved cylinder from the copy shown in FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an engraving head 20 constructed in accordance with theinvention and in position for engraving a gravure printing cylinder 22having a copper coating forming an outer surface 23. The engraving head20 preferably includes a cast metal housing 26 which rigidly supportsopposite end portions 27 and 28 (FIG. 2) of a shaft 30. The shaft 30includes an intermediate portion 32 which is integrally connected to theend portions 27 and 28 by torsional spring portions 33 of reduceddiameter. The intermediate shaft portion 32 extends through a tubularsleeve 36, and an intermediate liner of resilient material 38, such assilicone rubber, is bonded to the intermediate shaft portion 32 and thesleeve 36 for supporting the shaft 32 to avoid any lateral movement orvibration.

A notch 41 is formed within the shaft portion 32 and supports anactuator arm 42 which is rigidly secured to the shaft portion 32 by aset of screws. A cylindrical transverse hole 43 (FIG. 5) is formedwithin the actuator arm 42, and an elongated rod-like holder 44 isdisposed within the hole 43. A cutting stylus 45, preferably formed ofdiamond, is cemented into one end of the holder 44 which has a flatsurface 47. The surface 47 is engaged by a screw 49 which is threadedinto a hole 51 extending outwardly from the shaft 32 through theactuator arm 42 and intersecting the hole 43. As shown in FIG. 6, thediamond stylus 45 includes angularly related flat surfaces 53 whichcooperate with a bottom flat surface to form V-shaped cutting edges 56.The actuator arm 42 has a hole 57 which aids in reducing the mass of thearm and in obtaining a desired air flow across the stylus for removingchips.

Referring to FIGS. 2 and 3, an armature 58 is rigidly secured to theshaft 32 and projects outwardly between a pair of opposingelectro-magnets 61 which are mounted within the base portion of thehousing 26 and cooperate with the armature 58 to define air gaps 63.When the magnets 61 are energized by an alternating current, theintermediate shaft portion 32 and actuator arm 42 oscillate through amaximum arc of approximately 0.25 degree and at a frequency preferablybetween 3,000 and 5,000 cycles per second. This oscillation results invibrating or moving the cutting stylus 45 by a maximum distance of about100 microns.

The length and the stiffness of the torsional spring portions 33 of theshaft 32 are selected to provide the entire oscillating assembly with apredetermined rotational stiffness, preferably having a naturalfrequency of 2,000 to 3,000 cycles per second. It has been founddesirable for the operating frequency to be significantly greater thanthe natural frequency of the oscillating assembly in order to prevent ahigh resonant rise at the operating frequency, and thereby provide formore precise control of the vibratory movement of the cutting stylus. Inaddition, factors such as the operating temperature, the hardness of thecopper coating 23, the sharpness of the cutting stylus 25, etc. havemore affect on the control of the movement of the cutting stylus and thedepth of the engraved cells in a system with a high resonant rise.

As mentioned above, the layer 38 of resilient material connecting theshaft portion 32 to the rigid sleeve 36 provides the oscillatingassembly with a lateral stiffness which effectively presents lateralvibrations. In addition to these vibrations within the shaft 30 causedby the reaction of the cutting stylus 45 against the copper surface 23,there are also vibrations of the fundamental mode. To minimize suchvibrations, the intermediate shaft portion 32 may also be supported withintermediate support rods 64 (FIG. 5) which are rigidly connected to theshaft 32 at right angles and are also rigidly connected to the housing26. The rods 64 provide the shaft 30 with high lateral stiffness in X-Ydirections while also providing for high frequency oscillation of theshaft. The support rods 64 substantially eliminate the friction whichwould be produced by an intermediate support bearing and also minimizehysteresis which results in irregular, unpredictable motion of thecutting stylus 45.

It has been found that the support rods 64 should be of good steelspring material, having a cross-sectional area of about 0.02 square inchand a length of about 0.5 inch in order to provide the desired stiffnessand flexibility without fatigue. While two support rods 64 areillustrated in FIG. 5, it is apparent that additional sets of supportrods may also be used and, if desired, the torsional spring supportportions 33 at opposite ends of the shaft may be replaced bycorresponding sets of support rods 64. When the elastic, essentiallynon-compressible material 38 is used to provide the shaft 30 with a highdegree of lateral stiffness, substantially all of the oscillatoryassembly is preferably encompassed with the material to cover as largean area as possible and thereby provide the maximum lateral stiffness.

Referring again to FIGS. 2 and 7, undesirable transverse and rotationalvibrations in the oscillating assembly are also reduced by a dampener 65which includes a metal hub member 66 mounted on the shaft 30 between thearmature 58 and lower spring portion 33. A set of arcuate pads 68 ofresilient, non-compressible material is bonded to the hub member 66 atcircumferentially spaced intervals, and a corresponding set of arcuateweight or mass elements 69 are bounded to the pads 68 so that theelements 69 are free for limited movement relative to the shaft 30 andalso relative to the housing 26 of the engraving head.

The arrangement of the pads 68 and elements 69 provides for absorbingenergy without introducing undesirable hysteresis as would result if theresilient pads 68 were also bonded directly to the support housing 26.Such hysteresis causes both short and long term drifts in the depth ofthe cells which are engraved, and it may take several seconds for thehysteresis effects to disappear. The spring constant of the combinedelastic pads 68 and weight elements 69 may be selected so that theresonant or natural frequency is approximately the same as the resonantor natural frequency of the shaft assembly.

The frequency to be dampened by the dampener 65 shown in FIG. 7 isprimarily a single frequency established by the inertia of theoscillating assembly working against the torsional spring forcesproduced by the shaft portions 33. Generally this frequency is about2,000 to 3,000 cycles per second, but may be readily varied by changingthe spring constant or mass element 69. Since the weight or masselements 69 move about 180 degrees out of phase with respect to the hubmember 66 and at a greater amplitude, they absorb more energy for agiven area of dampening material than would the material if it was alsobonded to a large mass or directly between 66 and the housing.

FIGS. 8-10 illustrate diagrammatically different modifications ofdampeners which are also constructed in accordance with the invention.In each modification, it is desirable for the elastical or resilientelements and mass elements to be uniformly arranged around the shaft toavoid translational forces being introduced to the shaft, but it is notnecessary for the elements to be in the form of a continuous ring. Inthe modification shown in FIG. 8, the dampener includes a set of arcuatepads 73 which are bonded to the hub member 66 and are formed of aresilient material having embedded therein heavier particles 74 such aslead shot. The mass particles 74 act in the same manner as the masselements 69 towards dampening undesirable lateral and rotationalvibrations.

In the modification illustrated in FIG. 9, the pads 73 are also attachedor bonded to a stationary plate 76 which is rigidly secured to thehousing 26 of the engraving head. The attachment of the pads 73 to thehousing with a dampening material reduces the resonant rise of "Q" ofthe spring-mass system. The pads 73 may be made with a soft attachmentto plates 76 to minimize hysteresis in the oscillating components orassembly of the engraving head. In FIG. 10, the mass elements 69 of thedampener 65 are attached to the housing plate 76 through another set ofresilient pads or elements 68 and forms a dampener which functions inthe same manner as the dampener shown in FIG. 9.

Referring to FIG. 2, a guide shoe 80 is preferably formed of diamond andhas a flat face 81 (FIG. 4) which engages the copper surface 23 of thecylinder 22 to be engraved. The guide shoe 80 is cemented or secured toan elongated bar 84 which is secured to the upper portion of a plate 86.The plate 86 is secured to the housing 26 by a three point adjustablemounting which includes a screw 87 adapted to provide for slightpivoting or tilting of the plate 86. A set of adjustment screws 88 and89 provide for adjusting or tilting the plate 86 about the connectingscrew 87 so that the flat surface 81 of the shoe 80 may be preciselyaligned parallel with the axis of the cylinder 22. The guide shoe 80 ispositioned (FIG. 4) so that its leading edge 92 is spaced closely to thecylinder surface 23, for example, within one to three microns, so thatthe chips produced during engraving by the stylus 45 do not pass betweenthe face 81 of the guide shoe 80 and the cylinder surface 23 and resultin raising the shoe and stylus. As shown in FIG. 1, a suction hose 94 isconnected to an opening within the back wall of the housing 26 forremoving the chips from the stylus 45 and actuator arm 42 as the chipsare produced. Commonly, the chips are from three to fifty microns insize.

An inverted U-shaped yoke member 95 (FIGS. 1 and 2) is pivotallyconnected to the housing 26 at pivot pins 97 and carries a generallytriangular plate 99 which is secured to the yoke member 95 by a threepoint mounting formed by a set of adjustment and locking screws 102. Adiamond cleaning or scraping shoe 105 is mounted or cemented to anelongated bar 106 which is secured to the plate 99, and the shoe 105 hasan inclined shearing edge 107 (FIG. 4) which engages the cylindersurface 23 immediately after it is engraved by the stylus 45 to removeany burrs which may be produced on the surface by the engraving stylus.By adjusting the angular position of the plate 99, the edge 107 of theshoe 105 may be precisely positioned parallel with the axis of thecylinder 22. The cleaning edge 107 is urged against the cylinder surface23 by a spring (not shown) connected to the yoke member 95.

As also shown in FIG. 1, the engraving head 20 is supported for tiltingmovement by a set of leaf springs 110 which connect the engraving headto a support carriage (not shown) supported for both traversing movementparallel to the axis of the cylinder 22 and transverse movementperpendicular to the axis. A flat bar 112 projects rearwardly from thebase of the engraving head 20, and the bar 112 is coupled to thecarriage by a manually adjustable screw mechanism (not shown) whichprovides for manually tilting the engraving head to move the guide shoe80 and stylus 45 into engagement with the cylinder surface 23 by aspring (not shown) under the bar 112. In addition, a rotary cam member116 is positioned above the bar 112 and is eccentrically mounted on theshaft 117 of a stepping motor 118. When the motor 118 is energized, theengraving head 20 is automatically and slowly moved or tilted to movethe guide shoe 80 and stylus 45 into and out of engagement with thecylinder surface 23.

Referring to FIG. 11, the original copy which is to be engraved into thecylinder 22, is mounted on a copy support cylinder 125 supported forrotation at a predetermined constant speed. The copy is scanned by ascanning head (not shown) which includes a copy illuminating anddetecting system 130. In the system 130, light rays from a light source132 project through a center hole within a lens 134 and then past a mask136 and through a lens system 137 which focuses the light rays at anillumination point f1. The light rays which pass through the lens 134are deflected so that they are focused at an illumination point f2 whichis spaced a small distance from point f1, for example, approximately0.05 inch.

The copy on the cylinder 125 is located between the illumination pointsf1 and f2 at a pick-up point f3 which is the focal point of thecombination of the lens system 137 and a minor correcting lens 139. Therays passing through the mask 136 and lens 139 are reflected byreflectors 142 and 143 to an optical detector 145. A light shield 177 isused to control the light which enters the detector 145.

By using the two light illumination points f1 and f2 on each side of thenormal position of the copy surface f3, the higher illuminationintensity at the points f1 and f2 compensates for the falloff of thelight reflected to the detector 145 as the copy surface being scannedmoves from the point f3 between the illumination points closer to one ofthe illumination points. Thus the detector does not receive a falsereading due to irregularities in the surface of the copy when itssupporting cylinder is rotated. As a result, a radial change in the copysurface, for example 0.03 inch, results in a negligible change in thevoltage output of the detector 145. While there are several methods ofproducing the two focus illumination points f1 and f2, the systemillustrated in FIG. 12 has produced desirable results by producingextremely small illuminated spots and illumination at an angle whichavoids undesirable surface reflection. In addition, the small pickuppoint f3 is fully illuminated around 360 degrees to assure that auniform pickup signal is obtained between the illumination points f1 andf2.

As mentioned above, it is highly desirable to obtain accurate control ofthe cell depth with rapid depth changes and also to provide forproducing the maximum number of engraved cells per second. This requiresoperating the diamond cutting stylus 45 at a frequency which is abovethe natural frequency of the assembled oscillating components. Thus theresponse to a depth signal which is at the natural frequency of theengraver system must be multiplied by the "Q" or resonant rise of theoscillating components within the engraving head. The dampening systemsdiscussed above are helpful, but too much dampening and resultinghysteresis would be required to obtain a system which has a flatfrequency response through the resonant frequency of the oscillatingcomponents.

FIG. 12 shows a control system for operating the engraving head aboveits natural frequency. The control signal, which is derived from theoptical detector 145 within the scanner head, is used to engrave cellswhich have a depth proportional to the signal amplitude, irrespective ofthe frequency component of the signals. In accordance with theinvention, the control signal is passed through a band reject filterwhich may be of the R, L or C type or it may be of the active filtertype. However, it is necessary for the filter to be such that itsfrequency and "Q" characteristics may be adjusted to reduce the resonantfrequency response to the inverse characteristic of the engraving headfrequency response. The processed signal is directed along with a cellformation signal to a power amplifier which drives the coils 61 withinthe engraving head. As a result of the control system, the signalproduced by the detector 145 for controlling the diamond stylus is aflat frequency response when it passes through the natural frequency ofthe assembly of the oscillatory components. One or more additional bandreject filters may be used if the harmonic frequencies between thenatural frequency and the signal frequency presents a problem.

In reference to FIG. 13, the engraving of repeat patterns on a gravurecylinder usually requires the copy to be made so that it is longer thanone repeat length. The copy C illustrated in FIG. 14 has a joint J whichis manually made to be as invisible as possible. It is desired toengrave the image from line L1 to line L2 around the gravure cylinder ina manner such that an invisible joint is made between lines L1 and L2.Usually, there is a gray scale shift due to the photographic processwhich makes the copy in the area of line L1 darker or lighter than thecopy in the area of line 12. Thus when placed next to each other on theengraved cylinder, the joint of lines L1 and L2 becomes very visible,and it is expensive to correct this problem manually after engraving.This problem is effectively corrected by the control system illustratedin FIG. 14. The scanner scans the entire length of the copy shown inFIG. 13 as the copy is rotated on the copy support or scanner cylinder.The memory circuit remembers at least one signal pick up line from lineL1 to L2 and supplies a signal to the amplifier for driving the engraverhead so that line L1 is engraved next to line L2.

The scanner signal is fed into an integrator which samples a portion ofthe line such as the length A which may be from 1/4 inch to 1 inch. Astart pulse operates to start the memory circuit and to start theengraving cycle and cause the sample and hold amplifier to sample inline length A. At the end of the copy scanned at line L2, a separatesignal causes a second sample line length B of the scan line to besampled by the sample and hold amplifier. Any difference between the twosample signals is fed through an averaging network to a signal decaynetwork. When a start pulse is received by this network, it sends asignal to the amplifier which causes the reading from line L1 on thecopy to be increased or decreased to match the signal from line L2 onthe copy. Thus the engraving depth for each length just after L1 andjust before L2 are made to match.

The signal from the signal decay network decreases as the engravingproceeds from line L1 to line L2 in a gradual manner so that it is notapparent on the engraved surface on the printing cylinder. The grayscale shift from line L1 to line L2 on the copy is normally a large areaphenomena. If full correction is taken on a line-by-line basis, theremay be too much or too little correction taken which is due to thedetails in the copy. In such a case, it is desirable to use theaveraging network which averages several differential signals overseveral engraved lines, providing a more accurate area correction forgray scale errors.

It may be desirable for the engraver to operate so that its engravingstart (for line L1) is not the same as the start of the scan, the startpulse for the signal decay network would then be the start of theengrave signal. It is apparent that in this case, an engraving startpickup separate from the scanner may be used in this system. Inaddition, the exact location of the pickup signal may be varied as longas the pickup is representative of differences of intensity or gray inthe areas of the two lines to be joined.

While the methods and forms of apparatus herein described constitutepreferred embodiments of the invention, it is to be understood that theinvention is not limited to the precise methods and forms of apparatusdescribed, and that changes may be made therein without departing fromthe scope and spirit of the invention as defined in the appended claims.

The invention having thus been described, the following is claimed: 1.Apparatus adapted for engraving the surface of a gravure printingcylinder, comprising an engraving head, means supporting said head forrelative movement adjacent the surface, said head including a shaft andan engraving stylus, means on said shaft and supporting said styluseccentrically of the axis of said shaft, means supporting said shaft andsaid stylus for oscillation, power operated drive means for oscillatingsaid shaft and said stylus, a copy scanner having a light source,optical means for concentrating light rays on the copy being scanned andfor also detecting the light rays reflected by the copy, and saidoptical means being effective to produce two spaced illumination focalpoints and a copy pickup focal point spaced between the illuminationpoints.
 2. Apparatus as defined in claim 1 wherein said optical meanscomprise a lens system having an annular portion for producing anillumination focal point and a center portion for producing a copypickup focal point.
 3. Apparatus adapted for engraving the surface of agravure printing cylinder, comprising an engraving head, meanssupporting said head for relative movement adjacent the surface, saidhead including a shaft and an engraving stylus, means on said shaft andsupporting said stylus eccentrically of the axis of said shaft, meanssupporting said shaft and said stylus for oscillation, power operateddrive means for oscillating said shaft and said stylus, a copy scannerhaving a light source and a light detector, optical means including alens system having an annular portion surrounding a center portion, oneof said portions of said lens system cooperating with said light sourceto concentrate light rays on the copy being scanned, and the other saidportion of said lens system cooperating with said light detector fordetecting the light rays reflected by the copy.
 4. Apparatus as definedin claim 3 and including means for scanning a copy having a repeatingpattern adapted to form a joint, means for sensing the copy density atdifferent circumferentially spaced locations on the copy, means forcomparing the density at the different locations, means for introducinga correction for differences in density, and means for controlling saiddrive means to cause the engraving at the joint to be corrected forunbalance in copy density at the locations.
 5. Apparatus as defined inclaim 3 wherein said optical means are effective to focus the light raysat a plurality of spaced illumination points adjacent the copy. 6.Apparatus as defined in claim 3 wherein said optical means produces twospaced illumination focal points and a copy pickup focal point spacedbetween the illumination points.
 7. Apparatus adapted for engraving thesurface of a rotating gravure printing cylinder, comprising an engravinghead, means supporting said head for relative movement adjacent thesurface, said head including a shaft and an engraving stylus, means onsaid shaft and supporting said stylus eccentrically of the axis of saidshaft, means supporting said shaft and said stylus for oscillation,power operated drive means for continuously oscillating said shaft andsaid stylus while the printing cylinder is rotating, a copy scannerhaving a light source and a light detector, optical means forconcentrating light rays from said light source onto copy supported by arotating copy cylinder with the copy having a repeating pattern adaptedto form a joint, said optical means directing light rays from the copyto said light detector, means associated with said copy scanner forsensing the copy density at different circumferentially spaced locationson the rotating copy, means for comparing the density at the differentlocations, means for introducing a correction for differences in copydensity, and means for controlling said drive means to cause theengraving at the joint to be corrected for unbalance in copy density atthe different locations.